In optical networks comprising nodes interconnected by wavelength-division-multiplexed (WDM) fiber links, low frequency tones are often used for providing both power monitoring and identification of optical channels. For example, low-frequency tones each of a frequency between 10 kHz and 1 MHz may be used to modulate the amplitude of individual optical carriers at the edge of the network. To monitor the carrier power in a fiber link, an optical tap may be used to extract a measured sample of the propagating optical signal comprising wavelength-multiplexed carrier signals. The tapped optical signal is processed to detect the individual modulating tones, determine their frequency content, and measure the power levels of individual tones. Tapped-signal processing may be realized using low cost circuitry.
Methods and apparatus for identifying channels in an optical network are described in the following patent applications:                U.S. Pat. No. 7,054,556, titled “Channel identification in communications networks”;        U.S. Pat. No. 7,127,165, titled “Method and system for compensating for side effects of cross gain modulation in amplified optical networks”;        U.S. Pat. No. 7,139,486, titled “Method and apparatus for optical add/drop multiplexing in optical networks”; and        U.S. Pat. No. 7,142,783 titled, “Method and system for identification of channels in an optical network”.        
The presence of light-energy scattering in a WDM link results in crosstalk among the carrier signals sharing a WDM link. Accordingly, the measured power level of a specific tone, modulating a specific carrier signal, may reflect contributions of more than one carrier signal and, hence, may not accurately represent the power of the specific carrier signal.
There is a need, therefore, to explore methods for accurate estimation of the power levels of carrier signals in the presence of crosstalk in a WDM fiber link while still relying on processing a low-frequency envelope of a tapped optical signal. Accurate power-level estimation enables control of optical-power level across an optical network.